A variety of techniques are used to determine the presence and quantities of hydrocarbons (oil and gas) in earth formations. These methods are designed to determine formation parameters, including the resistivity, porosity, and permeability of the rock formation surrounding the wellbore drilled for recovering the hydrocarbons. Typically, the tools designed to provide the desired information are used to log the wellbore during drilling of the wellbore (referred to as measurement-while-drilling (MWD) or logging-while-drilling (LWD) or after drilling of the wellbore.
Certain formation evaluation (FE) measurements, such as Nuclear Magnetic Resonance (NMR) measurements and Neutron Density measurements, are sensitive to the actual formation temperature of the formation region of the measurements made by such tools. For example, signal amplitude of nuclear magnetic resonance (NMR) measurements for determining total porosity are inversely proportional to the temperature of the earth formation from which the NMR signals originate. For example if the absolute temperature Θ of the measurement region (also referred to as the “selected region,” sensitive region” or “sensitive volume”) of an NMR tool is Θformation=350K(77° C.) and the drilling fluid or “mud” temperature is Θmud=330K(57° C.), the error introduced in NMR data by using the mud temperature instead of the formation temperature will be about
      (          1      -                        Θ          mud                          Θ          formation                      )    =            (              1        -                              330            ⁢                                                  ⁢            K                                350            ⁢                                                  ⁢            K                              )        ≈          6      ⁢              %        .            
The actual formation temperature Θformation in the sensitive volume of the downhole NMR tool may not generally be the “virgin” formation temperature Θformationvirgin because the drilling fluid or “mud” penetrates into the formation surrounding the wellbore and thus the sensitive region. The change in the formation temperature is a function of the mud temperature Θmud (t) and the time elapsed between the time of drilling past the sensitive region and the time measurements are made. The elapsed time is a function of the drilling rate (rate of penetration (ROP) of the drill bit into the formation) and the offset between the drill bit and the sensors making the measurements.
Therefore, there is a need for estimating or determining the actual temperature of the sensitive region at the time the formation evaluation measurements are made. The disclosure herein provides apparatus, methods and computer programs for estimating the actual temperature of the sensitive region and using the estimated actual temperature to estimate or compute one or more properties of interest of the formation.